A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
U.S. Pat. No. 4,011,011 describes an example optical arrangement for a lithographic apparatus, which is known as a Micralign system. As can be seen from, for example, FIG. 3 of U.S. Pat. No. 4,011,011, this arrangement uses a primary concave mirror arranged concentrically with a secondary convex mirror to project light from a mask onto a wafer. The light is directed from the mask towards the primary concave mirror by a flat mirror arranged at 45 degrees to the optical axis of the primary concave mirror, and is then directed away from the primary concave mirror towards the wafer by a second flat mirror arranged at 45 degrees to the optical axis of the primary concave mirror.
While lithographic apparatuses having an optical system such as, for example, that described in U.S. Pat. No. 4,011,011, work well for their intended purposes, there is a present demand for lithographic apparatuses capable of producing a larger exposure field, especially for the manufacturing of active matrix thin film transistor liquid crystal displays.